1. Field of the Invention
This present invention pertains to the storage and transport of gas and liquids, and more particularly to storage and/or transport of refrigerated and pressurized fluid cargos (e.g., natural gas and natural gas liquids) in relatively thin-walled containers at low temperatures where commercial cargo densities can be achieved at moderate pressures and where the containers efficiently fill available storage and/or transport space.
2. Description of the Related Art
Gases are presently stored in relatively long cylinders, bullet tanks, oblate spheroids or spheres that all feature at least one circular cross-section cut and have relatively high wall thickness to diameter ratios (t/D) to resist pressure-induced hoop stresses because the materials used to fabricate these essentially rigid containers are of limited effective strength in the orientation of the critical stresses (regardless whether metallic or fiber-based matrix shells or hybrid composites thereof). The general structure of these containers is a rigid form having a circular cross-section.
The most common form of large-scale, pressurized gas containment for storage or transport purposes is based on the use of single wall metallic cylinders or bottles, which are often made of high-strength steels that may possibly contain moderate proportions of alloying elements, such as nickel, that provide a low-temperature-toughness characteristic. It is possible that the cylindrical shape will be of an extreme length-to-diameter ratio, taking the form of a pipeline whether straight or looped or coiled. See U.S. Pat. Nos. 5,803,005; 5,839,383 and 6,003,460, issued to Stenning et al. and U.S. Patent Application Pub. No. 20040216656 filed by Fitzpatrick et al. and assigned Ser. No. 10/472,615, each of which is incorporated by reference and collectively referred to as “the Stenning patents.” Sea NG Management Corp. of Calgary, Canada provides compressed gas storage and transportation services via coiled pipe under the trademark “Coselle,” which is understood to be based on the technology described in the Stenning patent documents. However, steel cylinders/pipeline gas storage containers discussed in prior art are relatively heavy as compared to the weight of the gas cargo contained. When the gas is to be transported (e.g., by truck, rail, barge, or ship), it is important to optimize the weight of the gas cargo to the weight of the containment because high containment weight generally means higher costs and limitations on how many containers can be placed in or on conveyances of limited load capacity (e.g., trucks on highways).
Some inventors have sought to improve the ratio of pressurized cargo mass to container mass by using such low temperatures that cargo can be liquefied at relatively moderate pressures. See, for example, U.S. Patent Application Pub. No. 20030183638, filed by Minta et al. and assigned Ser. No. 10/396,895 and U.S. Pat. No. 6,047,747, issued to Bowen et al.; U.S. Pat. No. 6,085,528, issued to Woodall; U.S. Pat. No. 6,460,721, issued to Bowen et al.; and U.S. Pat. No. 7,147,124, issued to Minta et al., each of which is assigned to ExxonMobil Upstream Research Co. of Houston, Tex. (collectively referred to as “the ExxonMobil patents”). See also U.S. Pat. Nos. 6,932,121; 6,964,180; and 7,155,918, each of which is issued to Shivers, III and assigned to ATP Oil & Gas Corp. of Houston, Tex. Each of the following patents and patent applications are incorporated by reference: U.S. Pat. No. 6,584,781, issued to Bishop et al., U.S. Pat. No. 6,655,155, issued to Bishop, and U.S. Pat. No. 6,725,671, issued to Bishop, and U.S. Patent Application Pub. Nos. 20020046547, filed by Bishop et al. and assigned Ser. No. 09/943,693 and 20030106324, filed by Bishop et al. and assigned Ser. No. 10/316,475, each of which is believed to be assigned to Enersea Transport, LLC of Houston, Tex., and are collectively referred to as “the Bishop patents” or “Bishop.” The Bishop patents consider that the mass ratio for circular cylinders can be commercially optimized by selecting storage pressures that minimize the compressibility factor Z for temperatures below about −10° C. while keeping the cargo stored as a dense phase fluid. U.S. Pat. Nos. 3,232,725 and 3,298,805 are incorporated by reference, each of which issued to Secord et al. and are collectively referred to as “the Secord patents” or “Secord.” Secord suggests an approach that can achieve very high mass ratios by allowing a mixed phase fluid to exist in the cargo containers by selecting temperature-pressure combinations that keep the cargo fluid state very close to the phase boundary for temperatures ranging from just below the critical temperature of methane to a minimum of about −200° F. (−129° C.). However, all of the above reference the application of their design principles to tanks of circular cross-sections to manage hoop-like stresses induced in the skins of their essentially rigid containers by the internal pressure of the cargos they store and/or carry.
U.S. Pat. No. 6,863,474, issued to Webster et al., describes using the external pressure supplied by the hydraulic head acting on a storage container placed deep beneath the surface of the sea to counter any internal pressure associated with storing natural gas as a means to reduce the required thickness of the shell of a cylindrical container. However, such a container is still expected to be an essentially rigid cylinder and the premise is little different than just laying a long loop of a large diameter pipeline deep undersea for storage.
Some have sought to reduce the weight of natural gas cargo containment cylinders (particularly, highly pressurized gas) by proposing or using concepts for composite wall structures (see American Society of Mechanical Engineers Pressure Vessel Codes and ASME PV Code Case 2390), including the following:                “built-up walls” using multiple layers of circular formed steel plates;        metallic base cylinder wrapped with very high strength metallic wire, wire mesh, or bands;        metallic, metallicized plastic or plastic base cylinder/liner wrapped with fiber-resin straps/bands, mesh, or reinforcement; and        metallic, metallicized plastic or plastic base cylinder/liner with a resin-based matrix of randomly-oriented short fibers.        
When external materials are laid over metallic base cylinders, auto-frettage (where a permanent pre-stressing of the liner is imposed by pressuring it up against the cured/bonded over-layers until it yields and plastically deforms) may be used to optimize the stress distribution between dissimilar materials under expected loadings. In all cases, the resultant containment cross-section is essentially circular and rigid. Auto-frettage is an engineering standard practice and is clarified within ASME Pressure Vessel Code Sect. VIII Div. 3, KD-5, “Design Using Auto-frettage.”
While individual fibers used for reinforcement may be of ultra-high strength glass or carbon fiber materials, compound wall structure is generally fabricated to provide resistance against stress induced by longitudinal and bending loads—not just hoop stress caused by the internal pressure of the stored content.
The composite wall cylinders have proved to be successful strategies for improving the gas/container weight ratio, but they have also proven to be a substantially more costly solution as compared to the simple metal cylindrical bottle (or pipeline) for storage of gas at a given pressure. In the case of simple metal cylindrical tanks, higher strength materials are chosen to limit the weight and cost impact of the containment.
The ExxonMobil patents describe ways to use plates with a strength of about 900 megapascals (900 MPa) based on the chemistry and steel-making processes for the plate material and composite wall construction methods to produce large diameter liquid cargo containers of relatively light weight as compared to previous practice. However, all the containers considered feature circular cross-sections on at least one plane of cut. The composite structures described have circular cross-sections so that fiber winding may be efficiently practiced.
In the Bishop patents, it was determined that the most efficient way to store large volumes of CNG in cylinders is in simple metallic cylinders but at subzero temperatures such that commercially attractive volume compression ratios (ratio of the volume of the cargo fluid at ambient conditions to the volume of stored cargo fluid at storage conditions, ranging from 250-350:1) can be achieved by using approximately half the storage pressure applied in ambient or elevated temperature storage concepts. U.S. Pat. No. 6,725,671, a Bishop patent, and U.S. Pat. No. 7,137,260, issued to Perry and assigned to Zedgas, Inc. of British Columbia, Canada, also recommend the mixing of heavier hydrocarbon gases or carbon dioxide to improve transport efficiency. This approach allows the “storage efficiency” (gas weight/container weight) to be increased dramatically without requiring the complexity and cost of a composite cylinder wall structure. However, Bishop's approach specifies that gas should be stored in a dense phase state to avoid liquid drop out that occurs under two-phase storage conditions. Pressures needed to maintain dense phase fluid storage at the targeted operating temperatures still push toward the use of very many strong, rigid cylinders with relatively thick, heavy walls when considering storage or transport of commercially significant quantities of natural gas. The heavy cylindrical containers are costly to support and transport whether being carried in or on land vehicles or marine vessels.
Except for lined carbon fiber composite tanks, moderate and high pressure gas cargo tanks as currently in practice or as cited in prior art exhibit relatively high self weights that impose practical constraints on the means of transports. Roadway load limits restrict the potential for using large diameter high pressure CNG tanks in land transport, even by rail. Practical ship designs that recognize realistic draft limitations for port entry and dry-docking are limited in how many of the heavy cylindrical cargo containers can be carried onboard.
When the cylinders are relatively short (i.e., with length over diameter ratios <<1000), as opposed to a coiled tubing concept as proposed in the Stenning patents, a very large number of cylinders must be linked with interconnecting piping and valves to create a large volume storage facility. Interconnecting and operating storage with many cylinders involves investment in large quantities of piping, connections, and controls.
Whatever the quantity of rigid cylinders or spheres required in the facility or onboard the means of transport, a substantial amount of space around each cylinder should be provided to allow inspection according to industrial standards. See ABS Guideline for Vessels Intended to Carry Compressed Natural Gases in Bulk, 2005. As a result of allowing standardized clearances around each cylinder in the cargo hold space of a marine vessel, cylinders of 1.0 meter (1 m) diameter would occupy only about 40% of the available footprint when spaced out practically with a vertical orientation. The larger the diameter of the cylinders, the larger the proportion of the available space that can be utilized. For example, a single cylinder of a diameter allowing just the minimum code-required clearance from the surrounding bulkheads could occupy about 70% of a square hold. However, it is unlikely that a single rigid cylinder designed to withstand the high pressures typically proposed in prior art at a diameter that would match the size of the space typically available within the hold of a typical ship could be practically fabricated from the materials and structures proposed by prior art, whether of single or composite skin construction. Therefore, it is expected that projects using cylinders of essentially rigid, circular cross-sections (straight or coiled) are likely to utilize substantially less than 70% of the cargo space available within the means of transport.
Therefore, a need exists for a safe cargo containment solution that allows better utilization of the cargo spaces within or on the means of transport (ship, railcar, or truck trailer) when transporting refrigerated and compressed fluids, especially, natural gas and/or liquefied natural gases of various hydrocarbon compositions.
The term “natural gas” as used in this document refers to light hydrocarbon compositions that are dominated by the methane molecule, but may be comprised of heavier hydrocarbon molecules as well as limited non-hydrocarbon impurities (such as water, carbon dioxide, and nitrogen) in any proportion that would exist as gas vapor at ambient temperature and pressure. This “natural gas” may have originated as a naturally occurring fluid stream extracted from the earth or as synthetically combined mixture of molecules created for the purposes of transport in or on some form of mobile platform (such as a ship, railcar, or truck trailer). “Compressed natural gas” may be referred to simply as “CNG”, whether refrigerated or not. “Pressurized liquid natural gas” or “pressurized natural gas liquid”, often referred to as “PLNG” herein, is deeply refrigerated, but may not necessarily be stored at temperatures below the critical temperature of methane.